1. Field of the Invention
The present invention relates to forward-looking radar imaging systems. In particular, it relates to radar imaging systems that use digital-beam-forming techniques to measure the spatial frequency components of a scene in the cross-track direction.
2. Description of Related Art
Conventional forward-looking imaging radars are widely used for aircraft-landing and vehicle-navigation applications. In such systems, a mechanically gimbaled antenna aperture is usually mounted on the front of an aircraft or other vehicle, and it is generally used for both transmit and receive functions. The antenna is pointed at a fixed elevation angle toward a spot in front of and below the vehicle. The mechanical gimbals allow the antenna to be scanned azimuthally along a cross track, perpendicular to the direction of motion of the vehicle. For each azimuth position along a cross-track scan, a radio-frequency pulse or series of signal waveforms is transmitted from the antenna and scatters off of targets in the illuminated area with some of the scattered energy returning to the antenna. Objects closer to the antenna will return an echo before those that are farther away. Thus, dividing radar returns into time bins based on the timing of the echo return is equivalent to dividing them into range bins reflecting the distance to the scattering target. The maximum resolution of the imaging radar in the along-track direction thus depends on how precisely this range gating can be performed, and this is largely a function of the transmit waveform bandwidth.
In the cross-track, or azimuth dimension, the resolution is primarily dictated by the size of the aperture, with a larger aperture creating a smaller beam footprint that is scanned along the cross-track direction. As the antenna is scanned cross track, pulses or radar waveforms are transmitted at the pulse repetition frequency (PRF) to acquire a series of returns from each of the beam footprints along the cross track scan length. Thus for each scan, a two-dimensional image is constructed with pixels in the along-track direction resolved by range gating and pixels in the cross-track direction resolved by aperture size and the PRF rate.
One drawback of this conventional approach is that the antenna gimbals add weight and complexity to the system. The scanning process itself also adds aberrations due to the fact that the platform moves between the beginning and end of the scan. Furthermore, all spectral components of the scene are integrated in the direction of the scanning pencil beam, requiring a Fourier transform of the received signals before efficient spatial spectral filtering can be performed. Accordingly, it would be useful to provide a system that uses one or more fixed antennas, that can acquire an entire scene at one time, thereby reducing motion-induced aberrations, and that acquires spatial frequency spectra directly, improving signal-to-noise ratios and presenting data in a form readily amenable to spectral processing.